An arrangement for controlling the output power of a drive unit of a motor vehicle is disclosed in published German patent application 4,037,237. Here, for the open-loop control of the output power of a drive unit, a drive torque desired value corresponding to the driver's request is pregiven and, with the input of an engine torque desired value, the engine torque is adjusted by influencing output parameters of the engine while considering transmission ratios corresponding to the desired value of the drive torque. The engine torque desired value is determined on the basis of the transmission ratio, output torque desired value and measured output engine speed from a characteristic field. The computation of the desired engine torque takes place in such a manner that the output torque which adjusts is made available independently of the gear in which the transmission is placed corresponding to the accelerator pedal position.
The following situation results with an automatic transmission having a converter. The desired value of the output torque is determined from the output engine speed and the accelerator pedal position and requires a certain desired torque at the turbine wheel of the converter in dependence upon the ratio of the gear which has been shifted into place. To obtain this desired turbine torque, a specific desired engine torque is required at the converter input in dependence upon the converter amplification. In order to obtain an output torque independent of the gear position and converter amplification, the converter amplification must be included in the determination of the desired engine torque. A concrete procedure for determining the converter amplification is not provided in the above-mentioned published German patent application.
In the electronic control of transmissions, it is known to compute a theoretical desired acceleration of the motor vehicle from engine torque, gear position and converter slip which is directly connected to the converter amplification. The slip of the converter can then be determined from a characteristic field as a function of the engine speed and engine torque.
The use of methods of this kind for determining the converter slip or the converter amplification resulting therefrom is however not suitable when the engine torque is unknown or when a desired value for the engine torque must be computed in advance in order to obtain the pregiven drive torque. Therefore, the necessary engine torque cannot be made available in all cases in a satisfactory manner with a view to the desired output torque value representing the request of the driver. An exact determination of the desired engine torque in accordance with the methods outlined above would further have an iterative operating procedure with corresponding complexity as the result since the converter slip or the converter amplification are dependent on the engine torque which is a variable which is sought.